The present invention relates to an electric power steering apparatus in which a motor is used as a source of a steering assist force.
Steering of an automobile is carried out by transmitting rotating operation of a steering wheel disposed within a vehicle room to a steering mechanism disposed on an outside of the vehicle room for steering wheels (front wheels, in general) for steering.
FIG. 1 is a sectional view of an electric power steering apparatus of prior art and FIG. 2 is a sectional view of a reduction mechanism and a motor portion.
As shown in FIG. 1, the electric power steering apparatus for the automobile has a first steering shaft 102 connected to a steering wheel 101, a torque sensor 105 for detecting steering torque based on a relative displacement between the first steering shaft 102 and a second steering shaft 104 connected to the first steering shaft 102 through a torsion bar 103 in a rotating direction, and the reduction mechanism 109 including a worm 107 and a worm wheel 108 for increasing a rotating force of a steering assisting motor 106 driven based on a detection result of the torque sensor 105 and for transmitting the rotating force to the second steering shaft 104. By means of the above constitution the electric power steering apparatus assists operation of the steering mechanism according to rotation of the steering wheel 101 by rotation of the motor 106 to reduce labor and a load imposed on a driver for steering.
Opposite end sides of the worm shaft 110 provided with the worm 107 and the second steering shaft 104 provided with the worm wheel 108 in shaft length directions are respectively supported by bearings 111, 112, and 113, 114 and movements of the shafts in diameter directions and the shaft length directions are prevented.
If the worm 107 and the worm wheel 108 are used as described above, a distance between rotation centers of the worm shaft 110 and the worm wheel 108 is preset in assembly. In this presetting, dimensional errors of the worm 107, the worm shaft 110, the bearings 111 and 112 for supporting the worm shaft 110, the worm wheel 108, the second steering shaft 104 for supporting the worm wheel 108, and the like cause variations in the distance between the rotation centers, backlash occurs after assembly, and noise due to the backlash is heard in the room of the automobile.
A meshing state changes with time because cogs of the worm 107 and the worm wheel 108 wear as a result of steering or because the worm wheel 108 made of synthetic resin contracts due to low temperatures during wintertime and the like and occurrence of the backlash is inevitable.
In order to reduce variations in the distance between the rotation centers in assembly, it is necessary to process parts such as the worm shaft 110, the bearings 111 to 114, the worm wheel 108, and the second steering shaft 104 with small dimensional errors, measure dimensions of all the parts, select the respective parts based on the measured dimensions, and assemble the parts. Therefore, costs of processing and assembly and cost of the entire electric power steering apparatus increase. Moreover, because the movements of the assembled worm shaft 110 and worm wheel 108 in directions intersecting the rotation centers are prevented, the distance between the rotation centers cannot be adjusted and the backlash cannot be adjusted when the meshing state has changed with time.
It is an object of the present invention to provide an electric power steering apparatus in which the above problems can be solved.
In an electric power steering apparatus according to a first invention for transmitting rotating force of a steering assisting motor to a worm shaft having a worm and being rotatably supported in a gear housing through a bearing and to a steering shaft provided with a worm wheel meshing with the worm and for assisting steering, the electric power steering apparatus has a bearing case provided with a holding hole in which the bearing is fitted and held, and a mechanism for decentering the bearing toward the worm wheel.
In the first invention, because the bearing with which the worm shaft is fitted can be moved toward the worm wheel, it is possible to easily set a distance between rotation centers of the worm shaft and the worm wheel without being influenced by dimensional errors of parts such as the worm shaft and to reduce costs of processing and assembly of parts as compared with the above-described prior art. Moreover, because the bearing case has the decentering mechanism, it is possible to easily provide the decentering mechanism. Because the bearing can be decentered in a state in which the bearing is fitted and held in the holding hole of the bearing case, the distance between the rotation centers can be adjusted satisfactorily when a meshing state of the worm and the worm wheel changes with time due to increase in wearing amounts of cogs of the worm and the worm wheel or contraction of the worm wheel made of synthetic resin as a result of low temperatures during wintertime and the like.
In an electric power steering apparatus according to a second invention, the decentering mechanism has an inclined face formed on an inner peripheral face of the holding hole such that the holding hole is inclined with respect to the rotation center of the worm shaft, and has an inclined contact surface provided on an outer periphery of the bearing and comes in contact with the inclined face.
In an electric power steering apparatus according to a third invention, the contact surface is provided on the outer peripheral face of a ring member fitted with and fixed to the outer periphery of the bearing.
In the second and third inventions, because of a simple structure in which the holding hole of the bearing case is inclined and the inclined contact surface is provided in association with the bearing, the structure of the decentering mechanism can be simplified to further reduce the cost.
In an electric power steering apparatus according to a fourth invention, a pair of bearings with which opposite end portions of the worm shaft are fitted respectively are provided, one of the bearings is held in the bearing case, and an elastic member for energizing the worm shaft toward the one bearing is provided on a side of the other bearing.
An electric power steering apparatus according to a fifth invention further comprises a mechanism for adjusting a deflection amount of the elastic member from outside.
In the fourth and fifth inventions, is it possible to satisfactorily prevent rattling of the worm shaft in the shaft length direction when adjusting the distance between the rotation centers of the worm shaft and the worm wheel.
In an electric power steering apparatus according to a sixth invention, the bearing case has a fitting portion rotatably fitted with the gear housing around the holding hole and the decentering mechanism decenters the holding hole with respect to the center of the fitting portion.
In an electric power steering apparatus according to a seventh invention, the decentering mechanism has a mechanism for rotating the bearing case from outside and changes a position of the worm shaft supported by the bearing fitted and held in the holding hole with respect to the worm wheel by rotating the bearing case.
In the sixth and seventh invention, because the bearing can be decentered by rotating the bearing case, it is possible to easily adjust the distance between the rotation centers in assembly. Moreover, it is possible to easily adjust the distance between the rotation centers when the meshing state of the worm and the worm wheel changes with time due to wearing of cogs of the worm and the worm wheel as a result of steering or contraction of the worm wheel made of synthetic resin as a result of low temperatures during wintertime and the like.
In an electric power steering apparatus according to an eighth invention, a pair of bearings with which opposite end portions of the worm shaft are fitted are provided, one of the bearings is held in the bearing case, and an elastic member is provided between the other bearing and a fitting hole in which the other bearing is fitted.
In the eighth invention, it is possible to satisfactorily adjust the distance between the rotation centers if a deflection amount of the worm shaft increases.
In an electric power steering apparatus according to a ninth invention, the worm shaft is connected to an output axis of the motor through a universal coupling.
In the ninth invention, it is possible to satisfactorily adjust the distance between the rotation centers if the deflection amount of the worm shaft increases.
In an electric power steering apparatus according to a tenth invention for transmitting rotating force of a steering assisting motor to a worm shaft having a worm and being rotatably supported in a gear housing through a bearing and to a steering shaft provided with a worm wheel meshing with the worm and for assisting steering, a hole portion of the gear housing for fitting with and holding the bearing is formed of a pair of split members formed by splitting the portion into two along a longitudinal direction of the fitting hole, one split member being movable toward the worm wheel with respect to the other split member, a member for displacing the one split member to change a position of the worm shaft supported by the bearing with respect to the worm wheel, and an elastic member provided between the other split member and the bearing are provided.
In an electric power steering apparatus according to an eleventh invention, the displacement member has a member for connecting the pair of split members, and an energizing member disposed between the connecting member and the one split member and having elastic recovery force greater than elastic recovery force of the elastic member.
In an electric power steering apparatus according to a twelfth invention, the connecting member is a clamp screw.
In an electric power steering apparatus according to a thirteenth invention, a pair of bearings with which opposite end portions of the worm shaft are fitted respectively are provided, one of the bearings is held in the bearing case, and the other bearing is a double-row angular contact ball bearing.
In an electric power steering apparatus according to a fourteenth invention, the worm shaft is connected to an output axis of the motor through a universal coupling.
In the tenth to fourteenth inventions, because by operating the displacement member, it is possible to deflect the elastic member, move the one split member, and decenter the bearing toward the worm wheel, the distance between the rotation centers can be adjusted easily.